One.relative_identifier(
  1 = 2,
  key: 3 = 4
) = Two.relative_identifier(
  2 = 1,
  key: 4 = 3
)
One.relative_identifier(
  1 = 2,
  key: 3 = 4
)(
  4 = 5,
  key: 6 = 7
) = Two.relative_identifier(
  2 = 1,
  key: 4 = 3
)(
  5 = 4,
  key: 7 = 6
)
One.relative_identifier key: 1 = 2
One.relative_identifier unqualified 1 = 2,
                                    key: 3 = 4
One.relative_identifier 1 = 2,
                        key: 3 = 4